1. Field of the Invention
The present invention relates to polarizing optical elements.
2. Background Art
Polarizers
The simplest grid element polarizers are devices that consist of a grid of parallel conducting wires. When light impinges on the grid, each orthogonal component of the radiation interacts differently with the wire grid polarizer. The component of the field which is parallel to the wires drives conduction electrons along the length of each wire, thus generating a current. The electrons in turn collide with lattice atoms, imparting energy to them and thereby heating the wires. In this manner energy is transferred from the field to the grid. In addition, electrons accelerating parallel to the wires radiate in both the forward and backward directions. The incident wave tends to be canceled by the wave reradiated in the forward direction, resulting in little or no transmission of that component of the field. The radiation propagating in the backward direction simply appears as a reflected wave. In contrast, the component of the wave that is perpendicular to the wires is essentially unaltered as it propagates through the grid. (Eugene Hecht, Optics, Chapter 8, pp. 333-334, Addison Wesley, San Francisco (2002)).
In 1960 George R. Bird and Maxfield Parish, Jr. described a grid polarizer for use in the microwave range having 2160 wires per mm (G. R. Bird and M. Parrish, Jr., J Opt. Soc. Am. 50:886 (1960)). Accordingly to the article, this was accomplished by evaporating a stream of gold (or aluminum) atoms at nearly grazing incidence onto a plastic diffraction grating replica. The metal accumulated along the edges of each step in the grating to form thin microscopic “wires” whose width and spacing were less than one wavelength across.
Grid polarizers have been developed for use both in the IR and visible spectrum. For example, U.S. Pat. No. 6,122,103 describes setting forth a wire grid polarizer for the visible spectrum which has a plurality of elongated elements supported on a substrate. U.S. Pat. No. 5,748,368 also describes a wire grid polarizer that polarizes light in the visible light spectrum.
Lithography
In order to create faster and more sophisticated circuitry, the semiconductor industry continually strives to reduce the size of the circuit elements. The circuits are produced primarily by photolithography. In this process, the circuits are printed onto a semiconductor substrate by exposing a coating of radiation sensitive material to light. The radiation sensitive material is often referred to as a “photoresist” or just resist. Passing the light through a mask, which may consist of a pattern of chrome or other opaque material formed on a transparent substrate, generates the desired circuit pattern. The mask may also be formed by a pattern of higher and lower regions etched into the surface of a transparent substrate, or some combination of the two techniques. Subsequent thermal or chemical processing removes only the exposed or only the unexposed regions of the resist (depending on the material) leaving regions of the substrate bare for further processing which in turn produces the electronic circuit.
Polarization at the reticle affects the lithographic performance of the lens several ways. First, the interaction of the illumination with features of the reticle, say for example dense lines of chrome, varies with polarization. The result is that the transmission and scattering of the mask depend on the polarization of the light and the features of the mask. Second, reflections at the surfaces of the lenses and mirrors are polarization dependent so that the apodization and to a lesser degree the wavefront at the projection optics (“P.O.”) depend on polarization. Also, the reflection from the surface of the resist depends on polarization, and this too is effectively a polarization dependent apodization. Finally, the rays diffracted from the reticle that are brought back together at the wafer must interfere to produce an image. However, only parallel components of the electric field can interfere, so the polarization state of each ray at the wafer affects the coherent imaging. Even with a perfect lens, the three dimensional geometry of the rays arriving at the wafer can reduce the contrast.
The primary reason for considering a polarized illuminator is to improve the image formed at the wafer by improving the interference of the diffracted rays at the wafer. This is particularly useful in high numerical aperture systems. Consider dipole illumination incident on a binary mask of dense lines. Each small region in the illuminator pupil (i.e., each pole of a low sigma dipole) is incoherent with other regions in the pupil and makes its own image at the wafer, so one can consider a single pole of the dipole illumination. The light diffracts from the reticle, and the dense lines produce tight diffraction orders. For small features, only two diffraction orders are accepted into the P.O. At the wafer, these diffraction orders recombine to form an image of the mask. The image of the mask depends on the contrast, and the contrast is dependent on the polarization.